The link redundancy method used in the RSTP ring improves the reliability and robustness of a network. The RSTP ring is a networking mode commonly used in Layer 2 aggregation layer in a metropolitan area network. With the increasing requirements of reliability for an access network, this networking mode is becoming the tendency for networking of access network.
As shown in FIG. 1, a link ring is formed by access layer equipments and aggregation layer equipments. It can be ensured that the communication link between each access equipment and aggregation equipment will not be interrupted even when a fault occurs at any point in the ring, thus significantly improving the capability of the access network in defending a single point fault.
The RSTP protocol is used as a topological control protocol in the RSTP ring. However, the application of the RSTP ring hampers the development of broadband video services based on the multicast technology, because the IGMP(Internet Group Management Protocol) is generally used as a control protocol of multicast video service between an access equipment and an aggregation equipment. In this case, in order to maintain the group membership of a program, it is necessary for the access equipment to transmit an IGMP REPORT packet to the aggregation equipment. There are at least two links in the RSTP ring between the access equipment and the aggregation equipment, and only one of the two links is available. As a result, the availability identification of the links becomes the first issue to be dealt with by the access equipment. If the IGMP REPORT packet is transmitted on the wrong link, the upper layer aggregation equipment will not receive the IGMP REPORT packet in a long period, and will consider that the access equipment no longer needs the program, so it will not transmit the multicast packets of the program to the access equipment.
Secondly, in order to prevent an unauthenticated multicast packet from being forwarded via the multicast table, it is necessary for the access equipment to check the source port and source IP address of an incoming multicast packet, to ensure that only the multicast packet transmitted from specific ports and specific video servers can be forwarded to the user port, and prevent an illegal multicast packet from disturbing the normal receiving and viewing of a user. While in the case of an RSTP ring, the port at which a server provides a specific program will vary dynamically, and if the port which is used by the access equipment to check the source port of a multicast program does not vary correspondingly, a normal video packet will not pass the checking and will be discarded.
For example, in the schematic networking diagram as shown in FIG. 2, an RSTP ring is constituted by one aggregation equipment (hereinafter referred to as AG) and two pieces of access equipment (e.g. access equipment A and access equipment B, hereinafter referred to as AXA and AXB, respectively). After the RSTP protocol is enabled, a port “AXB.0/5” with lower priority on the access equipment B (AXB) is blocked depending on the calculation of the spanning-tree. At this time, the aggregation equipment (i.e. AG) is a root bridge, the access equipment A (i.e. AXA) communicates with the aggregation equipment (AG) via a port AXA.0/7 which is the root port (the port for communicating with the root bridge) of the access equipment A (AXA). An IGMP REPORT packet will be transmitted out via the port AXA.0/7 which is also the ingress port of all the video programs. The access equipment B (i.e. AXB) communicates indirectly with the aggregation equipment (AG) via port AXB.0/1(through the access equipment A), its root port is port AXB.0/1. An IGMP REPORT packet will be transmitted out via the port AXB.0/1 which is also the ingress port of all the video programs. When the direct link AG.0/1-AXA.0/7 between the aggregation equipment and the access equipment A is interrupted for some reason, the RSTP protocol will re-converge and re-enable the port AXB.0/5 on access equipment B. At this time, the access equipment A communicates with the aggregation equipment via a port AXA.0/4 which becomes the root port of the access equipment A. If the access equipment A still transmits an IGMP REPORT packet via the port AXA.0/7, the video stream sent from the aggregation equipment will not be received. If the port AXA.0/7 is still considered as the ingress port of a program when checking the source port (actually, the video program stream is transmitted from the port AXA.0/4), the normal video stream can not pass the checking. Similarly, the same problem may also occur on the access equipment B.
At present, in order to solve the above problem, an IGMP REPORT packet is transmitted to all the uplink ports when the IGMP REPORT packet is transmitted upwards, so as to avoid the availability identification of the uplink links. In addition, the source port will not be checked for a video packet. However, it can be seen from this method that,
1. the uplink port of an access equipment may also connect with other network equipments besides one aggregation equipment, and there may be a number of multicast groups on these equipments (generally, a number of content providers may provide video services, among which the same multicast address may be used). If an IGMP REPORT packet is transmitted to all the uplink ports without distinguishing the ports, other equipments will receive the IGMP REPORT packet and then transmit other programs to the access equipment (it shall be noted that these programs are different from the program requested by the access equipment, but they have the same multicast address), so the access equipment will receive unexpected multicast packets.
2. If an IGMP REPORT packet is transmitted to all the uplink ports without distinguishing the ports, it is necessary for the CPUs to process the IGMP REPORT packet, thereby increasing the burden of other uplink equipments.
3. If the source port of a program is not checked, unexpected multicast packets will be transmitted to the user, so the user is unable to watch the program normally.